The ability of T cells to mediate an immune response against an antigen requires two distinct signaling interactions (Viglietta, V. et al. (2007) Neurotherapeutics 4:666-675; Korman, A. J. et al. (2007) Adv. Immunol. 90:297-339). First, an antigen that has been arrayed on the surface of antigen-presenting cells (APC) is presented to an antigen-specific naive CD4+ T cell. Such presentation delivers a signal via the T cell receptor (TCR) that directs the T cell to initiate an immune response specific to the presented antigen. Second, various co-stimulatory and inhibitory signals mediated through interactions between the APC and distinct T cell surface molecules trigger the activation and proliferation of the T cells and ultimately their inhibition.
The immune system is tightly controlled by a network of costimulatory and co-inhibitory ligands and receptors. These molecules provide the second signal for T cell activation and provide a balanced network of positive and negative signals to maximize immune responses against infection, while limiting immunity to self (Wang, L. et al. (Epub Mar. 7, 2011) J. Exp. Med. 208(3):577-92; Lepenies, B. et al. (2008) Endocrine, Metabolic & Immune Disorders—Drug Targets 8:279-288). Examples of costimulatory signals include the binding between the B7.1 (CD80) and B7.2 (CD86) ligands of the APC and the CD28 and CTLA-4 receptors of the CD4+ T-lymphocyte (Sharpe, A. H. et al. (2002) Nature Rev. Immunol. 2:116-126; Lindley, P. S. et al. (2009) Immunol. Rev. 229:307-321). Binding of B7.1 or B7.2 to CD28 stimulates T cell activation, whereas binding of B7.1 or B7.2 to CTLA-4 inhibits such activation (Dong, C. et al. (2003) Immunolog. Res. 28(1):39-48; Greenwald, R. J. et al. (2005) Ann. Rev. Immunol. 23:515-548). CD28 is constitutively expressed on the surface of T cells (Gross, J., et al. (1992) J. Immunol. 149:380-388), whereas CTLA-4 expression is rapidly up-regulated following T-cell activation (Linsley, P. et al. (1996) Immunity 4:535-543).
Other ligands of the CD28 receptor include a group of related B7 molecules, also known as the “B7 Superfamily” (Coyle, A. J. et al. (2001) Nature Immunol. 2(3):203-209; Sharpe, A. H. et al. (2002) Nature Rev. Immunol. 2:116-126; Collins, M. et al. (2005) Genome Biol. 6:223.1-223.7; Korman, A. J. et al. (2007) Adv. Immunol. 90:297-339). Several members of the B7 Superfamily are known, including B7.1 (CD80), B7.2 (CD86), the inducible co-stimulator ligand (ICOS-L), the programmed death-1 ligand (PD-L1; B7-H1), the programmed death-2 ligand (PD-L2; B7-DC), B7-H3, B7-H4 and B7-H6 (Collins, M. et al. (2005) Genome Biol. 6:223.1-223.7).
The Programmed Death 1 (PD-1) protein is an inhibitory member of the extended CD28/CTLA-4 family of T cell regulators (Okazaki et al. (2002) Curr Opin Immunol 14: 391779-82; Bennett et al. (2003) J. Immunol. 170:711-8). Other members of the CD28 family include CD28, CTLA-4, ICOS and BTLA. PD-1 is suggested to exist as a monomer, lacking the unpaired cysteine residue characteristic of other CD28 family members. PD-1 is expressed on activated B cells, T cells, and monocytes.
The PD-1 gene encodes a 55 kDa type I transmembrane protein (Agata et al. (1996) Int Immunol. 8:765-72). Although structurally similar to CTLA-4, PD-1 lacks the MYPPY motif (SEQ ID NO: 236) that is important for B7-1 and B7-2 binding. Two ligands for PD-1 have been identified, PD-L1 (B7-H1) and PD-L2 (B7-DC), that have been shown to downregulate T cell activation upon binding to PD-1 (Freeman et al. (2000) J. Exp. Med. 192:1027-34; Carter et al. (2002) Eur. J. Immunol. 32:634-43). Both PD-L1 and PD-L2 are B7 homologs that bind to PD-1, but do not bind to other CD28 family members. PD-L1 is abundant in a variety of human cancers (Dong et al. (2002) Nat. Med. 8:787-9).
PD-1 is known as an immunoinhibitory protein that negatively regulates TCR signals (Ishida, Y. et al. (1992) EMBO J. 11:3887-3895; Blank, C. et al. (Epub 2006 Dec. 29) Immunol. Immunother. 56(5):739-745). The interaction between PD-1 and PD-L1 can act as an immune checkpoint, which can lead to, e.g., a decrease in tumor infiltrating lymphocytes, a decrease in T-cell receptor mediated proliferation, and/or immune evasion by cancerous cells (Dong et al. (2003) J. Mol. Med. 81:281-7; Blank et al. (2005) Cancer Immunol. Immunother. 54:307-314; Konishi et al. (2004) Clin. Cancer Res. 10:5094-100). Immune suppression can be reversed by inhibiting the local interaction of PD-1 with PD-L1 or PD-L2; the effect is additive when the interaction of PD-1 with PD-L2 is blocked as well (Iwai et al. (2002) Proc. Nat'l. Acad. Sci. USA 99:12293-7; Brown et al. (2003) J. Immunol. 170:1257-66).
Given the importance of immune checkpoint pathways in regulating an immune response, the need exists for developing novel agents that modulate the activity of immunoinhibitory proteins, such as PD-1, thus leading to activation of the immune system. Such agents can be used, e.g., for cancer immunotherapy and treatment of other conditions, such as chronic infection.